This application claims priority to an application entitled xe2x80x9cForward Power Control Apparatus and Method for Use in a Multi-Carrier Communication Systemxe2x80x9d filed in the Korean Industrial Property Office on Dec. 31, 1999 and assigned Ser. No. 99-68121, the contents of which are hereby incorporated by reference.
1. Field of the Invention
The present invention relates generally to forward power control in a mobile communication system, and in particular, to an apparatus and method for performing a forward power control operation in a mobile communication system supporting multiple carriers.
2. Description of the Related Art
In general, a CDMA mobile communication system such as a CDMA-2000 system uses orthogonal codes for channel separation. When the mobile communication system uses orthogonal codes in a channel environment where there is no multi-path, no interference occurs between the channels using the different orthogonal codes.
However, when the mobile communication system uses orthogonal codes in the multi-path channel environment, orthogonality is not maintained between the channels in the transmission signal components due to delay of the paths and some of the signals input to the respective fingers act as interferences. In practice, a signal-to-interference ratio (SIR) or a signal-to-noise ratio (SNR) is measured using the interference components, and utilized as useful information for forward power control. This is because if a reception SIR value of a mobile station (MS) is given, a base station (BS) can adaptively control transmission power of a signal transmitted to the mobile station. That is, when the reception SIR value of the mobile station is lower than a threshold, the base station judges that the mobile station is in a bad reception condition and then, increases transmission power of a signal transmitted to a specific mobile station. Otherwise, if the reception SIR value of the mobile station is higher than the threshold, the base station judges that the mobile station is in a good reception condition, and then, decreases the transmission power of the signal transmitted to the specific mobile station. Therefore, all in all, it is possible to optimize the power efficiency by reducing an amount of the base station transmission power allocated to a specific mobile station. As one example, shown in FIG. 1 is a system which can perform a forward power control operation by measuring the reception SIR value of the mobile station and then transmitting the measured results to the base station. The system of FIG. 1 is disclosed in U.S. Pat. No. 5,559,790, issued on Sep. 24, 1996, entitled xe2x80x9cSpread Spectrum Communication System and Transmission Power Control Method Thereforxe2x80x9d, the contents of which are hereby incorporated by reference.
The forward power control system of FIG. 1, however, needs to exactly measure interference (or noise) signal power in order to measure an exact SIR value. In particular, such needs arise in a communication system having a plurality of carriers (hereinafter, referred to as a multi-carrier communication system) such as a CDMA-2000 system. This is because in the multi-carrier communication system, the channels are separately influenced according to the carriers and thus, the interference signal power can also vary according to the carriers. It is apparent that the forward power control system of FIG. 1 is designed considering a system having a single carrier rather than multiple carriers. Therefore, it is difficult to use the structure of FIG. 1 for forward power control operation of a multi-carrier communication system.
In addition, since the forward power control system of FIG. 1 performs despreading, accumulation and squaring on a pilot channel signal and a specific channel signal in order to measure the SIR value, channel utilization efficiency is decreased. Specifically, in FIG. 1, a PN despread signal output from a multiplier 304 is multiplied in a multiplier 310 by an orthogonal code WN generated by an orthogonal code generator 306 and is output as an orthogonal code despread signal. The orthogonal code despread signal is accumulated for a predetermined time by an accumulator 311, squared by a square unit 312, and then provided to a second input end of an S/N ratio measuring unit 316. The signal applied to the second input end of the S/N ratio measuring unit 316 is a noise power signal. In order to extract this noise power signal, the orthogonal code generator 306 should necessarily generate a specific orthogonal code which is not used in the base station during modulation of the transmission signal. In addition, the forward power control system of FIG. 1 performs despreading, accumulation and squaring on a pilot channel signal in order to measure a signal power value.
Since a noise power value measured by the S/N ratio measuring unit 316 is relatively much larger than a signal power value in real environment, it is difficult to perform an exact power control operation.
It is, therefore, an object of the present invention to provide a forward power control apparatus and method for use in a multi-carrier communication system.
It is another object of the present invention to provide a forward power control apparatus and method for guaranteeing efficient utilization of channels in a multi-carrier communication system.
It is further another object of the present invention to provide an apparatus and method for calculating SIR by measuring signal energy and interference energy for a forward power control operation in a multi-carrier communication system.
It is yet another object of the present invention to provide an apparatus and method for measuring interference energy without generating a separate orthogonal code.
To achieve the above and other objects, a mobile station of the mobile communication system measures signal energy on a traffic channel and interference energy on a sync channel to calculate SIR, compares the calculated SIR with a threshold, and then transmits the comparison results to a base station for a forward power control operation.
To this end, a PN despreader multiplies a received signal by a unique PN code and outputs a PN despread signal. An orthogonal code despreader multiplies the PN despread signal by a sync channel orthogonal code and a traffic channel orthogonal code and outputs first and second orthogonal code despread signals. An interference energy measurer measures interference energy by processing the first orthogonal code despread signal. A signal energy measurer measures signal energy by processing the second orthogonal code despread signal. An SIR calculator calculates SIR depending on the measured signal energy and interference energy. The calculated SIR is compared with a predetermined threshold and the compared results are transmitted to the base station for the forward power control operation.
Preferably, the PN despreader comprises a PN code generator for generating the PN code; and a muliplier for multiplying the received signal by the PN code and outputting the PN despread signal.
Preferably, the orthogonal code despreader comprises an orthogonal code generator for generating the sync channel orthogonal code and the traffic channel orthogonal code; a first multiplier for multiplying the PN despread signal by the sync channel orthogonal code and outputting the first orthogonal code despread signal; and a second multiplier for multiplying the PN despread signal by the traffic channel orthogonal code and outputting the second orthogonal code despread signal.
Preferably, the interference energy measurer comprises an accumulator for accumulating the first orthogonal code despread signal; a subtracter for subtracting from each other values of two neighboring symbols in the signal accumulated by the accumulator; and a squarer for squaring the subtraction signal by the subtractor to measure the interference energy.
Preferably, the signal energy measurer comprises an accumulator for accumulating the second orthogonal code despread signal; and a power control bit extractor for extracting a power control bit from the signal accumulated by the accumulator and measuring a value of the extracted power control bit as the signal energy.
Further, the apparatus comprises a long code generator for informing the power control bit extractor of a position of the power control bit in the signal accumulated by the accumulator in order to enable the power control bit extractor to use information about the position in extracting the power control bit.
Preferably, the interference energy and the signal energy are measured in a unit of a power control group.